The roast/leach/electrowin process is the most important method of zinc production, accounting for approximately 80% of all zinc produced. In the process, zinc sulfide concentrate is converted to zinc oxide by roasting, then leached in sulfuric acid to form soluble zinc sulfate which is readily separated from impurities such as arsenic, antimony, copper, cadmium, cobalt and nickel which are adsorbed by the insoluble hydrous oxides formed in the leaching stage and can be further removed by the cementation process, and finally the solution is electrolyzed in an electrolytic cell where zinc metal deposits at the cathode. Oxygen is liberated at the cell anode, regenerating sulfuric acid which is recycled to leach further zinc oxide.
Zinc reduction and hydrogen reduction are competing processes in the electrolytic cell, hydrogen reduction being thermodynamically favored over zinc reduction. Zinc reduction can be kinetically favored however due to the high overpotential for hydrogen deposition on suitable metal surfaces. This can be done by conducting the solution purification stage to remove metals that promote hydrogen reduction such as cobalt and nickel, and by ensuring that the deposited zinc is always cathodically protected to prevent it from redissolving. A further need of the process is that the spent electrolyte returned from the cells to the leach step should have as high a sulfuric acid concentration as possible to achieve a high reaction rate while minimizing the size and investment cost of the leaching equipment. Optimal performance in terms of these considerations is achieved when the spent electrolyte has a sulfuric acid:zinc mole ratio of about 2.
In processes currently used, electrolysis is performed in cells with parallel plate electrodes, with aluminum for the cathode and various alloys as the anode. For considerations of energy consumption, the most efficient operation is achieved with a current density of approximately 400 amperes per square meter (A/m.sup.2) of cathode surface. In no such process does the current density ever exceed 1,000 A/m.sup.2. Because of this low intensity operation and the essentially two-dimensional electrode surface configuration, economic considerations require the use of numerous large electrolytic cells and thus entail a high investment cost. Furthermore, the cathode must be periodically removed from the cell to permit detachment of the zinc deposit and cleaning of the cathode, which require the operator to disconnect the circuit. A further difficulty with the conventional process is the emission of acid mist by the cell. The mist is an environmental hazard and difficult to contain.
A variation on this process that overcomes some of these difficulties is the use of an electrolytic cell with a particle bed electrode, i.e., a bed of particles in either intermittent or continuous contact with a current feeder, which is an electrified surface similar to one of the electrodes in a conventional cell, supplying the charge to the particles. Deposition of zinc takes place at the surfaces of the particles, which offer a much greater surface area per unit volume of cell than a simple plate cathode. Current density in a particle bed cell can be significantly decreased due to the greater effective surface area. This allows the process to be operated more intensely, with a higher interfacial current density between anode and cathode. Furthermore, the particles can be periodically or continuously withdrawn, thereby eliminating the need to remove the plate cathode and strip zinc deposit from its surface.
Three forms of particle bed electrodes have been disclosed--fluidized beds, stationary beds and moving packed beds. Fluidized bed electrodes suffer from the difficulty that some portion of the particles is at all times electrically isolated from the current feeder. These isolated particles tend to redissolve in the acid electrolyte, causing excessive generation of hydrogen gas at the cathode at the expense of zinc deposition. This lowers the current efficiency and energy efficiency of the cell.
In packed (stationary) bed electrodes, all particles are in constant contact with the current feeder, removing the difficulty of particle dissolution. Unfortunately, the depositing zinc causes the particles to agglomerate, making it difficult if not impossible to remove them from the cell on a continuous or intermittent basis.
Moving (or moving packed) beds are a hybrid of fluidized and stationary beds. Particle movement is maintained at a level that is high enough to prevent particle agglomeration yet low enough to keep void space to a minimum and to keep the particles predominantly in contact with the current feeder. A disclosure of moving bed electrolysis is found in Scott et al., U.S. Pat. No. 4,272,333, issued Jun. 9, 1981. Scott et al. address copper, zinc, cobalt and manganese deposition from various alkali, acidic and neutral solutions using an electrolytic cell in which particles in the solution move as a packed bed across the surface of an electrode. The patent reports high current efficiencies (the amount of current used in reduction of the metal as a percentage of the total current consumed in the cell) and low energy consumption for copper deposition, but for zinc deposition unfortunately the results are considerably less favorable. This is understandable in view of the greater reactivity of zinc and hence its greater tendency to dissolve in acid sulfate electrolytes.